1. Field of the Invention
This invention relates to fiber lasers and more specifically to a high power single-mode low-cost fiber laser formed from erbium and ytterbium co-doped phosphate-glass.
2. Description of the Related Art
Rare-earth doped glass fiber lasers were first proposed in the 1960s and have received considerable attention in the 1980s for potential applications in optical communication (Michel J. F. Digonnet, xe2x80x9cRare-Earth Doped Fiber Lasers and Amplifiers,xe2x80x9d Marcel Dekker, New York 2001). For laser emission to occur, the active fiber is placed inside a resonant cavity. The optical feedback can be provided simply by the reflectivity of the end facets, by mirrors, by distributed feedback Bragg (DFB) gratings, or by distributed Bragg reflectors (DBR), or by constructing a ring cavity structure. Laser emission occurs when the total gain overcomes the losses in the cavity. Hence, a minimum gain has to be achieved to reach the laser threshold condition.
Fiber lasers have many characteristics, among them are mode structure (transverse and longitudinal) and output power. The type of fiber, single-mode or multi-mode, which is used as the active material, dictates the transverse mode. The longitudinal mode spacing is determined by the length of the fiber cavity, the longer the fiber the narrower the mode spacing. Therefore, long resonator cavities have numerous longitudinal modes within the width of the gain spectrum. A wavelength selective reflector such as a Bragg grating can reduce the number of modes. Single mode (single wavelength) performance can be achieved using an ultra-short cavity of less than about 5 cm together with a wavelength selective reflector. Output power is dictated by the total gain, which is generally proportional to the length of the fiber. Thus, output power must typically be traded off against single-mode performance.
The most common fiber laser product is a high power multi-mode laser. Standard Erbium-doped Silica lasers require a cavity length of several meters to produce sufficient output power due to low doping concentrations. A. Claesson et al. xe2x80x9cNovel Er:Yb:phosphate glass fiber laser pumped by a 946 nm Nd:YAG laser,xe2x80x9d Conference on Lasers and Electro-Optics, 2001 has demonstrated a similarly high power multi-mode laser with a 22 cm Erbium-doped phosphate glass fiber. Claesson""s phosphate fiber has more gain per centimeter than does standard silica due to elevated doping concentrations.
Another class of products is a single-mode laser. For many optical applications such as wavelength division multiplexing (WDM), high power ( greater than 10 mW) single mode lasers are in demand. To deploy these lasers in volume in the burgeoning metro markets the lasers must be inexpensive.
Semiconductor lasers are the most prevalent in this class and by themselves are relatively inexpensive. However, a booster amplifier is required to produce output powers greater than 10 mW. The inclusion of the booster amplifier makes semiconductor lasers quite expensive and bulky.
U.S. Pat. No. 5,237,576, DiGiovanni et al. describes single mode fiber lasers of 5 cm or less using fluorine-phosphorous-doped matched index cladding, a germania-alumina-doped outer core and an alumina-erbium-doped inner core. The optical properties of this glass composition limit output power to 50 xcexcW at a pump power of 24 mW and exhibit a slope efficiency of only 0.25%. DiGiovanni""s laser would also require a booster amplifier to reach output powers in excess of 10 mW.
Southampton has demonstrated single mode performance in 1.5 cm phosphosilicate fiber lasers co-doped with Er:Yb and fabricated by the combination of MCVD (Modified Chemical Vapor Deposition) and solution doping processes (W. H. Loh et al. Journal of Lightwave Technology, Vol. 16, No.1, pp. 114-118 January 1998). Loh reports output power levels of 10-40 mW with a slope efficiency of approximately 25% at a single wavelength and a saturated output power level of about 60 mW. The output power attainable over an entire band such as the C-band and the saturated output power will be limited by the doping levels of Erbium, hence gain of the fiber that can be supported the phosphosilicate glass host. Doping levels of Er:Yb in phosphosilicate glass have been reported of 0.06:1.8 and 0.16:1.4 weight percent Er:Yb. (G. Vienne et al., xe2x80x9cFabrication and Characterization of Yb3+:Er3+ Phosphosilicate Fibers for Lasersxe2x80x9d Journal of Lightwave Technology, Vol. 16, No. 11, November 1998, pp. 1990-2001) Phosphosilicate glass will not support appreciably higher doping concentrations because the ions will cluster and cause quenching.
Existing single-mode fiber lasers may exhibit a problem with xe2x80x9cself-pulsationxe2x80x9d. To avoid self-pulsation, it is necessary to keep the Er3+ concentration low enough to reduce ion-pair quenching, which in turn reduces gain and output power. See J. R. Kringlebotn et al. xe2x80x9cHighly-efficient, low-noise grating-feedback Er3+:Yb3+ codoped fibre laserxe2x80x9d Electronics Letters Jun. 9, 1994, Vol. 30, No. 12. pp. 972-973; Francois Sanchez et al. xe2x80x9cEffects of ion pairs on the dynamics or erbium-doped fiber lasersxe2x80x9d Physical Review A, Vol. 48, No. 3, September 1993, pp. 2220-2229; J. L. Zyskind et al. xe2x80x9cTransmission at 2.5 Gbit/s over 654 km using an Erbium-Doped Fibre Grating Laser Sourcexe2x80x9d Electronics Letters Jun. 10, 1993, Vol. 29, No. 12, pp. 1105-1106; and Guillaume Vienne et al. xe2x80x9cFabrication and Characterization of Yb3+:Er3+ Phosphosilicate Fibers for Lasersxe2x80x9d.
Northstar Photonics has demonstrated high power, single mode performance in planar waveguides fabricated by ion exchange techniques in Er doped phosphate glasses (PCT Publication WO 00/52791 by M. P. Bendett entitled xe2x80x9cRare-Earth Doped Phosphate Glass Lasers and Associated Methodsxe2x80x9d). 2.2. cm long waveguide lasers formed in Yb/Er co-doped phosphate glass showed 168 mW of output power of single frequency with TE polarization with no mode hopping for 611 mW of launched pump power at 979 nm, corresponding to a 26% slope efficiency at that particular wavelength. The fabrication of planar waveguide lasers by ion-exchange technology requires expensive photolithography manufacturing techniques. Furthermore, the total doping concentration is constrained since the glass is doped by substitution of Yb and Er for Na, and the total doping concentration cannot exceed 10 wt %.
Deployment of an all-optical network into the metro market will require compact low-cost continuous single-mode lasers that can deliver greater than 10 mW of output power over the C-band and preferably greater than 50 mW.
In view of the above problems, the present invention provides a compact low-cost single-mode fiber laser with output power in excess of 50 mW over the C-band (1530 nm-1565 nm).
This is accomplished by co-doping a phosphate glass fiber with high concentrations of erbium and ytterbium (Er:Yb). The phosphate glass supports the high doping concentrations without self-pulsation that are required to provide sufficient gain per centimeter and the slope efficiencies needed to achieve high power in the ultra short cavity lengths necessary to support single-mode lasers. The use of fiber drawing technologies provides a low cost solution. Absorptive mode coupling facilitates multi-mode clad pumping of the ultra-short fiber, which further reduces the cost deploying fiber lasers in the burgeoning metro markets.
More specifically, the fiber is drawn from a phosphate glass preform that is doped with 0.5-5.0 wt. % erbium ions and 0.5-15.0 wt. % (single-mode core pumped) or 5-30 wt. % ytterbium ions (multi-mode clad pumped) to form a highly doped core surrounded by a phosphate cladding. At least one wavelength-selective reflector such as a grating partially defines an optical resonant cavity of 5 cm or less that encompasses the fiber and provides the feedback necessary to sustain lasing. A source of pump radiation illuminates the fiber to excite erbium and ytterbium ions and provide gain. The ultra-short cavity produces a mode spacing that is comparable to the wavelength-selective reflector""s line width so that the erbium lases at a single longitudinal mode. The high gain per centimeter produces slope efficiencies of at least 30% and modeled output powers in excess of 50 mW over the C-band.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: